1. Field of the Invention
The present invention relates to an eyepiece suited for use for example in telescopes and microscopes.
2. Related Background Art
Usually, such optical instrument as a telescope or microscope uses an eyepiece for further magnifying and observing a real image formed by an objective lens. Referring now to FIG. 5, there is illustrated an example of the conventional eyepieces.
This example shows an eyepiece including, in the order from the eyepoint (E.P) side, a first lens group G.sub.2 1 composed of a cemented lens of a negative lens and a positive lens and having a positive refractive power and a second lens group G.sub.2 2 composed of a single positive lens component. This eyepiece has a focal length f of 19.8 mm, an apparent field of view of 51.degree., an eye relief of 10.3 mm, a focal length f.sub.2 1 of the first lens group G.sub.2 1 of 31.9 mm, a focal length f.sub.2 2 of the second lens group G.sub.2 2 of 37.2 mm, and a spacing D of 8.9 mm between the first lens group G.sub.2 1 and the second lens group G.sub.2 2.
The following Table 1 shows the parameter values of this eyepiece. Here, r.sub.i represents the radius of curvature of a lens surface R.sub.2 i, d.sub.i the surface spacing on the optical axis between the lens surface R.sub.2 i and a lens surface R.sub.2 i+1, n.sub.1 the d-line refractive index of the medium between the lens surface R.sub.2 i and the lens surface R.sub.2 i+1, and .nu..sub.i the Abbe number of the medium between the lens surfaces R.sub.2 i and R.sub.2 i+1.
TABLE 1 ______________________________________ R.sub.i r.sub.i d.sub.i n.sub.i .nu..sub.i ______________________________________ R.sub.21 232.5 1.1 1.728 28.3 R.sub.22 15.5 6.7 1.603 60.6 R.sub.23 -17.0 8.9 R.sub.24 31.9 4.3 1.620 60.1 R.sub.25 -79.0 ______________________________________
With such conventional eyepiece having a relatively large field angle (e.g., an apparent field of view of over 40.degree.), a relatively large pupil aberration appears as will be seen from FIG. 6. To provide a fully satisfactory aberration compensation even for the peripheral portion of the visual field by such eyepiece gives rise to a problem that the addition of a compensation optical system for this purpose is required and the number of lenses used in the eyepiece system as a whole is inevitably increased. Particularly, the compensation of such aberrations as distortion aberration is difficult thus tending to result in a lens construction including 4 to 6 lenses.